Bone is a primary site of metastasis from prostate cancer (PCa) and most patients with advanced PCa experience complications from the bone lesions that are incurable. Despite important advances that have been made in PCa research, molecular mechanisms behind PCa bone metastases are not well understood. Emerging literature evidence suggests that conditions like obesity and inflammation, known to disturb homeostasis in the bone microenvironment, and contribute to bone destruction, may be contributing factors to colonization and growth of prostate tumors in the bone. The cysteine protease, cathepsin K (CTSK), is a major collagenase involved in osteoclastic bone resorption. In prostate cancer, CTSK levels are higher in bone metastases compared to corresponding primary tumors and soft tissues from the same patients. In addition, CTSK plays important roles in inflammation and is a newly merging marker of adiposity. Our previous studies have shown that adipocyte maturation is severely impaired in bone marrow of CTSK deficient mice. Progression of prostate tumors in the bones in delayed in the absence of CTSK and correlates with significant reduction in levels of two pro-inflammatory factors that affect tumor aggressiveness and osteoclast differentiation (i.e., CXCL1 and CXCL2), further underlining the role of this protease in inflammation-related tumor aggressiveness in the bone. Therefore, we hypothesize that growth and aggressiveness of metastatic prostate cancer in the bone is driven by the CTSK-expressing bone marrow adipocytes and the adipocyte- driven inflammation involving CXCL1 and CXCL2 chemokines. The project is composed of three specific aims. The Aim 1 is to investigate contributions of bone marrow-derived adipocytes to macrophage inflammation and the consequent increase in prostate tumor cell invasiveness and aggressiveness. The Aim 2 is to determine how adipocyte and macrophage-derived CXCL1 and CXCL2 affect tumor cell proliferation, invasion, extracellular matrix degradation and cytokine profiles in vitro, and how the changes in tumor cell aggressiveness ultimately drive macrophage phenotype in the bone tumor microenvironment. The Aim 3 is to assess direct effects of high fat diet on inflammation of the bone marrow and determine the consequences of the resulting pro-inflammatory state on tumor progression in the bone. These studies will utilize combination of in vivo and in vitro approaches including: tumor cell-adipocyte-macrophage co-culture system, in vivo knockout model of bone metastasis, immunoblotting, ELISA assays, cytokine arrays, RT PCR and Lentivirus knockdown technology. Changes in tumor cell proliferation, invasiveness, extracellular matrix degradation, cytokine profiles, and macrophage phenotype will be studied in response to CXCL1 and CXCL2 as well as potentially other adipocyte and macrophage-derived factors. These studies will validate the importance of CTSK-driven events in bone marrow inflammation and investigate CXCL1 and CXCL2 as novel signaling factors and potential targets for therapeutic intervention in bone metastatic disease. )